1. Field of the invention
The present invention relates to the field of vehicle tyre sensors.
2. Description of the related art
The incorporation of electronic devices within pneumatic tyres is taking a greater importance in order to increase the safety of vehicles. Tyre electronics may include sensors and other components suitable for obtaining information regarding the behavior of a tyre, as well as various physical parameters thereof, such as for example temperature, pressure, number of tyre revolutions, vehicle speed, etc.
Such information may become useful in tyre monitoring and/or alarm systems.
Furthermore, active control/safety systems of the vehicle may be based on information sent from sensor devices included within the tyres.
Active safety systems use information about the external environment of a vehicle to change its behavior in pre-crash time period or during the crash event, with the ultimate goal of avoiding a crash altogether. Initially, active safety systems were primarily focused on improving the vehicle longitudinal motion dynamics, in particular, on more effective braking Anti-lock Braking Systems (ABS) and Traction Control (TC) systems. TC systems prevent the wheel from slipping while improving vehicle stability and steerability by maximizing the tractive and lateral forces between the vehicle's tyre and the road. These systems were followed by more powerful vehicle stability control systems, e.g., Electronic Stability Program (ESP), Vehicle Stability Control (VSC), and Dynamic Stability Control (DSC). These latter systems use both brakes and engine torque to stabilize the vehicle in extreme handling situations by controlling the yaw motion. Active suspension systems are also an important part in vehicle active safety systems. They have been traditionally designed by trading-off three conflicting criteria: road holding, load carrying and passenger comfort. The suspension system has to support the vehicle, provide directional control during handling maneuverers and provide effective isolation of passengers/payload from road disturbances.
The active safety control systems described above are based upon the estimation of vehicle dynamics variables such as forces, load transfer, tire-road friction. The more accurate and “real time” the parameter estimation, the better the overall performance of the control system. Currently, most of these variables are indirectly estimated using on-board sensors, and are not very accurate. Using measurements made by sensors fitted on the vehicle tyres would provide far more accurate estimation of the parameters relevant to the vehicle dynamics.
Setting up a system based on sensors fitted on the vehicle tyres is however a challenging task, for several reasons.
The inside of a tire is a harsh environment experiencing high accelerations (at a speed of 200 Km/hr an acceleration equal to about 3 g is experienced inside the inner liner of the tyre), and cannot be reached without taking the tire off the wheel. This situation poses very difficult problems: the high centrifugal acceleration implies that the sensor be light weight, for example not to unbalance the tyre, robust and small.
The fact that the tyre moves continuously with respect to the body of the vehicle forces to choose a wireless communication link for the communications from/to the sensors. However, the communication environment in which the sensor devices and the receiver are located is very harsh: in the immediate vicinity of a sensor device the wheel rim and the wheel arch of the car's body form two large signal reflectors. Both these parts are typically in metal and are curved in such a way that they tend to reflect incident waves back into the area, confining them. Furthermore, the radius of curvature of these two vehicle parts is of the order of the wavelength used for wireless transmission, making reflections much more complex. Also, the sensor device is inside the tyre and has to transmit through the tyre in some way: a true line of sight communication channel cannot be achieved since the tyre, being composed of a metal mesh and rubber, attenuates the signal dramatically.
Another issue is connected to the sensors' power supply; replacing the sensors' batteries is impractical because of the difficulty of reaching inside the tire. Hence, it is of primary importance that the sensor devices power consumption be as low as possible.
As disclosed in the U.S. patent application Ser. No. 12/654,705 filed on 29 Dec. 2009 and assigned to the present Applicant, herein incorporated by reference, some of the above issues can be solved by adopting a communication between sensor nodes fitted on vehicles' tyres and a sensor coordinator device fitted in the body of the vehicle exploiting Ultra Wide Band (UWB) transmission for the uplink (from the sensor nodes to the coordinator) and a narrowband transmission—such as one of the so called Industrial Scientific and Medical (ISM) radio bands—for the downlink (from the coordinator to the sensor nodes). The adoption of such communication scheme is advantageous because it allows to exploit the advantages of the UWB transmission for the uplink and at the same time the advantages of the ISM transmission for the downlink. Specifically, UWB is a technology suitable for low-cost, low-power, short-range and high-throughput wireless data transmission, which is robust against inter-symbol interference due to multi-path interference and lack of line-of-sight communications. Moreover, ISM transmission allows to strongly reduce the power consumption at the receiver side (sensor node) for the downlink, guaranteeing at the same time a sufficient throughput (which, in the downlink case, is relatively low).
Thus, by employing the solution proposed in such patent application, each sensor node—and the sensor coordinator device as well—need to be equipped with proper antenna systems, capable of transmitting and receiving both in the UWB band and in the ISM band.
The design of an antenna system to be installed on a sensor node which is destined to be fitted into a vehicle tyre is a very complex task, due to all the previously mentioned problems. Indeed, also such antenna system has to operate in a very harsh environment, which is subjected to high accelerations and that is located in the immediate vicinity of metallic bodies acting as signal reflectors. Moreover, since the sensor nodes have to be light weight and small, the antennas to be installed thereon have to fulfill very strict constraints in term of size. This size constraints force to employ miniaturized antennas whose dimensions are significantly lower than the wavelengths used in transmission and in reception; however, an antenna of such type typically has very poor performances from the bandwidth point of view. Another issue regards the presence of metallic elements in the sensor nodes themselves, which strongly degrade the antennas' performances. Indeed, since each sensor node comprises transmitting and receiving circuits, sensor circuits and at least one power source, all the metallic elements included in such electronic circuits interfere with the antenna near field, altering the impedance matching and the radiation pattern. A still further problem regards the choice of the best location on the sensor nodes where to install the antennas; indeed, in order to improve their mechanical robustness, certain portions of the sensor nodes—such as the top and the bottom ends—are preferably to be left empty.
The issue of transmitting/receiving radio signals to/by devices fitted on a vehicle tyre has already been faced in the art. For example, most of the presently available Tyre-Pressure Monitoring Systems (TPMS) use Radio Frequency Identification (RFID) devices, which are adapted to communicate through Radio Frequency (RF) signals; however, such RF signals oscillate at relatively low frequencies (e.g., few hundred of MHz) with relatively narrow bands, and thus they are not in compliance with a transmission of the UWB/ISM type.
For example, in “RF-Design Characterization and Modeling of Tyre Pressure Sensors” by Brzeska, Pontes, Chakam and Wiesbeck, The 2nd European Conference on Antennas and Propagations, 2007, EuCAP 2007, an antenna for TPMS applications is disclosed, consisting of a load monopole operating at 430 MHz.
The U.S. Pat. No. 6,781,561 discloses an antenna for a tire pressure detector structured as a coil.
In “Double loop antenna for wireless tyre pressure monitoring” by Genovesi, Monorchio and Saponara, Electronics Letters, Vol. 44, Issue 24, Nov. 20, 2008, the antenna is of the printed double loop type.
The U.S. Pat. No. 7,250,914 discloses a composite antenna for a tyre, which antenna is structured as large loop and located near the edge of the tyre itself.
Other known antennas suited to be employed in devices fitted in a tyre are designed to operate at even lower frequencies (hundreds of KHz or few MHz), such as the antennas disclosed in “Design and development of a miniaturized embedded UHF RFID Tag for Automotive Tire Applications”, Ant. Tech. Small Ant.s and Novel Metam, 2006 International Workshop March 2006, in the European patent application No. EP1713021, and in the PCT patent application WO/1999/029522.
Antennas adapted to correctly operate at higher frequencies, having at the same time small size, have been recently studied. However, such antennas have been mainly designed to be employed in mobile communications handsets or similar applications, as it is disclosed in, e.g., “A folded loop antenna system for handsets”, by Morishita, Kim, Koyanagi and Fujimoto, 2001 IEEE Antennas Propag. Symp., vol. 3, in “Planar trapezoidal and pentagonal monopoles with impedance bandwidths in excess of 10:1”, by Evans and Ammann, Proc. IEEE Int. Symp. Antennas Propagation, 1999, and in “Broadband roll monopole” by Chen, IEEE Trans. Antenna Propag., vol. 51, no. 11.